A splitter filter for use with VDSL at the cabinet
J W Cook
The design of a service splitter filter for VDSL has been studied for the case of VDSL deployed at a remote node such as a
flexibility cabinet and where it is desired to apply VDSL signals over either telephony or ISDN signals originating in the
telephone exchange. Even taking into account all of the requirements of telephony, ISDN, VDSL, HPNA and RFI, it is shown
that a remarkably small and simple splitter will suffice.
1. Introduction
" return loss of line port with low-pass port terminated in
DSL technology follows the mould cast for ADSL in
Zm better than 18 dB from 200 Hz to 4 kHz,
Vits need for some form of diplexer or  splitter to
separate low frequency signals from the higher frequency
" a high impedance (> 1 k&!) at high frequencies
VDSL signals so as to enable two systems to operate over
(>1.1MHz) on the extension wiring side in order to
the same twisted wire pair. For VDSL the low frequency
have the minimum impact on home networking
signals may easily include ISDN as an alternative.
signals,
The splitter has many functions. It protects telephony
" a high impedance (> 1 k&!) at high frequencies
and/or ISDN generally from the high levels of higher
(>1.1MHz) on the network wiring side in order to
frequency energy used to transmit the VDSL signal. Equally
have the minimum impact on VDSL.
it must protect VDSL from the transients generated
primarily during the signalling events of telephony
Zm is the complex reference impedance consisting of
(dialling, ringing, ring trip, etc); it must also protect the
320 &! in series with 1050 &! in parallel with 230 nF.
VDSL system from the wild fluctuations in impedance and
Return loss requirements should be met with or without
even in linearity which may occur when telephones change
a VDSL modem connected.
operational state (e.g. from off-hook to on-hook).
Good balance, common-mode and linearity per-
The design of splitters has always been difficult,
formance and low resistance are also required along with
generating many performance compromises for the services
tolerance of certain levels of DC current and voltage, but
involved.
these are functions of implementation rather than of filter
prototype design.
This paper describes a VDSL splitter design process. It
includes a prototype design and its simulated and measured
3. ISDN requirements
performance.
he VDSL splitter should pass basic rate ISDN signals
Tthrough its low pass path. ISDN signals have a working
2. Telephony requirements
bandwidth of ~1 kHz to 40 100 kHz, in other words over
or a VDSL splitter the telephony requirements are less
an order of magnitude greater than that of telephony. It is
Fcritical than for ADSL because they are unlikely to be a
easier to accommodate such signals under VDSL
limiting factor in the design. Therefore, easily sufficient, but
(compared, say, with under ADSL) because VDSL itself
perhaps not necessary, telephony requirements targets can
occupies (or at least may occupy) a much higher frequency
be laid down such as those below without there being any
range than ADSL.
likely adverse impact on the splitter design:
In attempting to design the splitter low-pass filter to
pass ISDN signals, the ISDN requirements become
" insertion loss between Zm (complex) loads from 200
dominant over the telephony requirements and so must be
Hz to 4 kHz, <Ä…0.5 dB,
considered more carefully. Earlier work has identified the
" return loss of low-pass port with line port terminated in requirements to pass ISDN signals without any significant
Zm better than 18 dB from 200 Hz to 4 kHz, impact on performance: 173
BT Technol J Vol 19 No 2 April 2001
A SPLITTER FILTER FOR USE WITH VDSL AT THE CABINET
207.36 µH 231.04 µH line port
" insertion loss between 135 &! loads from 1 kHz to
80kHz, <Ä…0.5 dB,
1.65 nF
" return loss of low-pass port with line port terminated in
low-pass
135 &! better than 18 dB from 1 kHz to 80 kHz,
port
15 nF
" return loss of line port with low-pass port terminated in VDSL port
135 &! better than 18 dB from 1 kHz to 80 kHz.
Return loss requirements should be met with or without
a VDSL modem connected.
Fig 1 Optimised splitter topology.
4. VDSL requirements
The equivalent minimum single-ended filter topology
t is still early days for VDSL systems and there are many
identified was as shown in Fig 1.
Ioptions in the draft standards.
If VDSL is deployed at the cabinet (rather than at the It should be noted that only the DC-blocking capacitor
exchange) it is not as yet clear what low frequency signals for the high-pass path of the splitter is shown. Although a
are permitted. There is a design risk that, if VDSL uses complete high pass filter was included in the optimisation
frequencies already allocated to ADSL, it may not be this was done only to prove that such a design is possible
spectrally compatible. One design option in this case is to with the optimised DC blocking capacitance. In practice,
suppress VDSL signals below 1.1 MHz (the top of the the rest of the high-pass filter would normally be
ADSL band) to ensure spectral compatibility with ADSL. implemented as part of the VDSL modem and not really be
With this assumption the following are reasonable design considered part of the splitter.
objectives for the splitter VDSL channel:
6. Full splitter design
" line port to VDSL port insertion loss in 100 &! shall be
fleshed out balanced splitter design is shown in Fig 2.
less than Ä…0.5 dB from 1.1 MHz to 12 MHz,
AThe bell wire is  regenerated on the low-pass port side
" line port and VDSL port return loss to 100 &! when of the splitter. This is because extending the existing bell
other port terminated in 100 &! shall both be better than wire through the splitter would require extra filtering and/or
18 dB from 1.1 MHz to 12 MHz, would tend to by-pass the splitter low-pass function.
" low-pass port to VDSL port isolation should be >70 dB
A common-mode choke (CMC) has been introduced to
over the band 1 MHz to 12 MHz, with some relaxation
isolate the network and building wiring common-mode
permissible in the transition band (55 dB from 80 kHz
noise environments. This choke has been placed on the line
to 1.1 MHz).
port, which means it must pass the high frequency VDSL
signals at up to 12 MHz. This puts a severe design
5. Design method
constraint on the leakage inductance of this device;
he design method has been to use an optimisation however, for emissions reasons, a CMC is desirable in this
Tprogram to assess the performance of a prototype position.
splitter topology against all of the eleven target
The total space required by the splitter is of the order
requirements itemised in sections 2, 3 and 4.
1.5cu inches, but this is critically dependent on the wound
Starting parameter values for the components making
component construction, and smaller implementations may
up the prototype topology were derived from appeals to
well be possible.
conventional filter theory. The optimisation program can
then optimise the parameter values for the topology against
7. Simulated performance
the conflicting requirements (mainly over impedance for
telephony and ISDN) and taking into account the loading
hen simulated, the full splitter achieves all of the design
effect or not of the VDSL circuit on the low pass filter.
Wexpectations and targets set out earlier in the nominal
Preferred values can then be chosen for the components
case with two small exceptions. The target isolation
while keeping a check on the effects on all aspects of
performance is not quite met over a narrow frequency range
performance. Finally the worst-case effects of component
just above 1.1 MHz, while the target ISDN return loss is not
tolerances on all aspects of performance can also be
quite met just below 80 kHz. Neither of these limitations is
assessed, all within the same optimisation tool.
considered particularly severe. It has been found that a more
Using this approach a range of filter topologies have complex 5th order filter could meet the design targets fully
been investigated, with the objective of finding the simplest  however, the increase in complexity is not considered
174 topology that can meet all the requirements. justified for the small increase in performance.
BT Technol J Vol 19 No 2 April 2001
A SPLITTER FILTER FOR USE WITH VDSL AT THE CABINET
VDSL
ports
~180µH
207.36µH 231.04µH
polyswitch
15 nF
low-pass
250V
port
line ports
5%
polyswitch
1.8µF
ring
capacitor
ring
Fig 2 Full splitter circuit diagram.
7.1 Worst-case performance Equally an ISDN system was found to operate normally
over a channel connected through the low-pass path of the
splitter even on a long line.
The simulated worst-case performance of the splitter
due to tolerance spreads of 5% in the capacitor and wound
More detailed tests of system reach and noise immunity
component values has also been computed.
are pending.
In general, there is little degradation in performance
8. Conclusions
with the exception of ISDN return loss which then falls to
his short paper has collected together the basic signal
only 16 dB above 60 kHz. As this is still outside the Nyquist
Tprocessing requirements for a service splitting filter that
band for ISDN this should still be adequate.
can be used for telephony, or ISDN and VDSL, and gone on
to show that all the conflicting requirements can be closely
Worst-case simulations of the 5th order design showed
met in a rather simple and compact filter design.
that it was little better than the simple 3rd order design in
the worst case due to increased sensitivity to component
This situation is a considerable improvement on the
values.
ADSL splitter design problem that is full of nearly
intractable compromises. The simple, compact and highly
7.2 Measured filtering performance satisfactory filter for VDSL will do much to further the
early and trouble-free roll-out and adoption of VDSL-based
telecommunications services.
An instance of the splitter has been constructed and
found to function as expected from the simulations
according to network analyser measurements.
John Cook joined BT as a student in 1974,
and graduated in electronics from Queen
Mary College, University of London, in
7.3 Performance with VDSL, ISDN and telephony in
1978. Based at Martlesham he then worked
practice
on various aspects of speech codecs and
telephony analogue line interface design. In
1987 he joined a team developing an ISDN
The splitter was used in conjunction with a pair of basic rate transceiver chip, and later on many
other DSL development projects including
VDSL modems.
HDSL and ADSL. Later he became an
adviser on copper access systems with
particular emphasis on signal processing and
No impact on maximum range was observed. No errors
analysis. During this time he has made
were observed on the VDSL system due to telephony significant contributions to the study of
impulsive noise and its effects on digital access transmission systems. In
signalling activities. Telephony function including
recent years he has been contributing to the work on spectral compatibility
transmission and ringing appeared unaffected by the
in access networks and in particular its rationalisation for the purposes of
insertion of the splitter. local loop unbundling.
175
BT Technol J Vol 19 No 2 April 2001